1. Field of the Invention
The present invention relates to a method pertaining to a burner and to a burner-cooler combination.
More specifically, the invention relates to a method of reducing material wear in the operation of furnace-heating burners. The burner concerned may be a gas burner, an oil burner, or a solid fuel burner.
The invention is exemplified below with reference to a combined gas burner and cooler.
2. Description of the Related Art
Furnaces are often heated with the aid of a gas burner. This burner will typically consist of a gas source from which gas is led through a nozzle and then ignited to provide a flame. The extremely hot gases of combustion are led through an open-bottom inner pipe. The inner pipe is surrounded by a closed-bottom outer pipe so as to form a closed pipe system. The waste gases are led out through the upper part of the outer pipe. The pipe system is situated in the heated furnace volume and there contributes to the heating process by transferring the heat generated by combustion to the heated volume of the furnace, primarily by thermal radiation.
Because the internal volume of the pipe system is not in contact with the heated furnace volume, the combusted gas or its combustion products will never come into contact with the products to be heated in the furnace.
A temperature of 500-1100° C. is a typical working temperature of such a furnace.
When the product heated in the furnace is ready to be removed therefrom, it will, of course, take a considerable length of time for the furnace, its heated volume, and the heated product to cool down to a temperature at which they can be handled without the aid of special tools. In order to hasten the furnace cooling process, cold air is often pumped into the inner pipe so as to cool down the heated furnace volume actively, by transferring the heat stored in the heated furnace volume to the cold air via the outer pipe.
In this type of operation, the outer pipe of the gas burner is named a “cooling finger.” The cooling air is thus led through the same pipe system as that in which the gas burner is included while the gas and fuel supply is switched off during this process, and there is opened instead an air supply source from which cooling air flows through the pipe system. The cooling air flow will often have a very large volume. For instance, the volume of cooling air used when cooling the furnace is typically 100 m3/h.
At times, the arrangement is used solely as a cooling finger.
Since the temperature of the cooling air is, of course, much lower than the temperature of the atmosphere in the heated furnace volume, and since the cooling air does not communicate directly with said atmosphere, those parts of the pipe system that constitute the barrier between the cooling air and the heated furnace volume will be subjected to strong material stresses. These stresses are sufficiently powerful to cause significant mechanical stresses in the joints between the different parts of the pipe system constituting said barrier.
In the case of known techniques, the cooling air is led through the inner pipe that opens into the outer pipe which is in direct contact with the enclosed furnace volume and through which the cooling air further passes and exits from the pipe system. In the region where the cooling air exits from the inner pipe and turns and flows back through that part of the outer pipe volume that is not accommodated by the inner pipe volume, the cooling effect is relatively strong. Consequently, the stresses on the material resulting from the powerful temperature gradients that occur in the outer pipe material will be significant in this region, the bottom construction of the outer pipe being particularly subjected to such stresses. The bottom construction will often consist of a bottom plate which is firmly fixed mechanically to the outer pipe. Material wear in the joints between said bottom plate and the barrel surface of the outer pipe will be significant regardless of how the bottom plate is secured, i.e., by welding, screwing, etc., such wear also being caused by cyclically varying loads which lead to thermal fatigue.
Other solutions to these problems have been proposed. For instance, one solution proposes that the outer bottom plate is given a curved shape so as to accommodate more effectively those material stresses that are applied to the construction by the temperature gradients that occur in cooling finger operations.
However, none of the presented proposals has led to an effective solution to the problem. The outer pipe and/or the bottom plate must still be replaced relatively often, with unnecessary high maintenance costs as a result.
Consequently, it is desirable to find an outer pipe bottom construction that will minimize material wear resulting from the temperature gradients that occur in said construction, therewith reducing the maintenance requirements on the gas burner and increasing its length of life.